In recent years, a need for improved fuel consumption has led to development of a vehicle intended to improve fuel consumption by increasing the engine (internal combustion engine) compression ratio, and a vehicle intended to improve fuel consumption through downsizing and the use of turbocharging.
With an increase in the compression ratio, it is necessary to increase the output voltage of the ignition coil device, and also to increase the withstand voltage inside the ignition coil device.
In addition, a method for improving fuel consumption by mounting an auxiliary machine may be adopted. This results in an additional limitation imposed on the mounting space of the ignition coil device, so that there is also a need for reducing the size and weight of the ignition coil device.
Accordingly, there is a demand for a small ignition coil device having a high output voltage and a high withstand voltage.
An ignition coil device for an internal combustion engine may be configured, for example, by winding a primary coil and a secondary coil around the outer circumference of a center electrical steel core (center core), and arranging a side electrical steel core (outer circumferential core) on the outside thereof.
These components are accommodated in an insulating case, and the insulation is maintained by filling the space inside the case with an insulating resin or the like.
Some ignition coil devices have a structure in which the side electrical steel core is covered around with an elastomeric material in order to relieve thermal cycle stress, and other ignition coil devices include divided side electrical steel cores for the purpose of ease of assembly, or insert a magnet at the divided position of the side electrical steel cores in order to enhance the magnetic efficiency.
At the division end portions of the side electrical steel cores and the division end portions of the elastomeric material, electric field concentration or thermal cycle stress concentration occurs, which may result in a reduction in insulation caused by the electric field concentration, detachment caused by the stress concentration, a reduction in insulation caused by cracks, or the like. Although the structure in which the side electrical steel core is covered around with the elastomeric material is effective in relieving thermal stress, the voltage endurance property of the elastomeric material itself is lower than that of an insulating filler such as an insulating resin.
The interface between the insulating resin and the secondary bobbin around which the secondary coil is wound has a withstand voltage inferior to that of the insulating resin or the secondary bobbin.
Moreover, in the case where detachment occurs between the elastomeric material and the insulating filler, the electric field is increased in an air layer formed by the detached portion. Therefore, it is necessary to ensure an insulation distance for a portion where the electric field is concentrated or a portion with a weak dielectric strength.
There is also a case where insulation has been increased, for example, by reducing the wall surface height of the secondary bobbin in order to prevent a size increase and ensure insulation (see Patent Document 1).